WIP Apply Base Relocs For Webcil in Wasm CoreCLR

docs/design/mono/webcil.md

@@ -130,6 +130,13 @@ struct WebcilHeader {
 };
 ```
 
+#### Webcil Header (V1 Changes)
+For Webcil V1, the Reserved0 field may be used to store a 1-based index which corresponds to a
+base reloc section.
+```
+    uint16_t Reserved0; // 0, or 1-based index of .reloc webcil section
+```
+
 The Webcil header starts with the magic characters 'W' 'b' 'I' 'L' followed by the version in major
 minor format (must be 0 and 0).  Then a count of the section headers and two reserved bytes.
 
@@ -184,3 +191,18 @@ Lossless conversion from Webcil back to PE is not intended to be supported.  The
 documented in order to support diagnostic tooling and utilities such as decompilers, disassemblers,
 file identification utilities, dependency analyzers, etc.
 
+
+### Webcil V1 (Base Relocations)
+It is possible to specify base relocations in the standard PE base relocation format in Webcil V1.
+Relocations are grouped by page (default 4kb), and each relocation entry is a `uint16_t` containing:
+
+- Relocation Type (upper 4 bits)
+  - IMAGE_REL_BASED_DIR64 (wasm64)
+  - IMAGE_REL_BASED_HIGHLOW (wasm32)
+  - IMAGE_REL_BASED_WASM32_TABLE (wasm32)
+  - IMAGE_REL_BASED_WASM64_TABLE (wasm64)
+- Offset (lower 12 bits)
+
+`IMAGE_REL_BASED_WASM{32, 64}_TABLE` relocations represent a "table base offset" fixup; They should be used to indicate places
+where function pointer table indices need to be offset after the Webcil payload has been loaded by the runtime. The offset will
+be dependent on the state of the table when an implementation's loader loads a Webcil module.

src/coreclr/inc/webcildecoder.h

@@ -48,7 +48,7 @@ struct WebcilHeader
     uint16_t VersionMajor;   // 0
     uint16_t VersionMinor;   // 0
     uint16_t CoffSections;
-    uint16_t Reserved0;
+    uint16_t Reserved0;      // 1-based index of the base relocations section, or 0 if none
     uint32_t PeCliHeaderRva;
     uint32_t PeCliHeaderSize;
     uint32_t PeDebugRva;
@@ -137,7 +137,8 @@ class WebcilDecoder
 
     // Webcil is always flat, never mapped in the PE sense
     BOOL IsMapped() const { return FALSE; }
-    BOOL IsRelocated() const { return FALSE; }
+    BOOL IsRelocated() const { return m_relocated; }
+    void SetRelocated() { m_relocated = TRUE; }
     BOOL IsFlat() const { return HasContents(); }
 
     // ------------------------------------------------------------
@@ -151,7 +152,7 @@ class WebcilDecoder
     BOOL HasNTHeaders() const { return FALSE; }
     CHECK CheckNTHeaders() const;
     BOOL Has32BitNTHeaders() const { return FALSE; }
-    BOOL HasBaseRelocations() const { return FALSE; }
+    BOOL HasBaseRelocations() const;
     BOOL HasWriteableSections() const { return FALSE; }
     BOOL HasTls() const { return FALSE; }
 
@@ -258,6 +259,7 @@ class WebcilDecoder
     TADDR                m_base;
     COUNT_T              m_size;
     BOOL                 m_hasContents;
+    BOOL                 m_relocated = FALSE;
     const WebcilHeader  *m_pHeader;
     mutable IMAGE_COR20_HEADER *m_pCorHeader;
 };

src/coreclr/pal/inc/rt/ntimage.h

@@ -1163,6 +1163,15 @@ typedef IMAGE_BASE_RELOCATION UNALIGNED * PIMAGE_BASE_RELOCATION;
 #define IMAGE_REL_BASED_ARM_MOV32             5
 #define IMAGE_REL_BASED_THUMB_MOV32           7
 
+#ifdef FEATURE_WEBCIL
+//
+// Webcil-specific based relocation types.
+// The fixup adds a WASM table base offset to the value at the relocation address.
+//
+#define IMAGE_REL_BASED_WASM32_TABLE          12
+#define IMAGE_REL_BASED_WASM64_TABLE          13
+#endif // FEATURE_WEBCIL
+
 //
 // Archive format.
 //

src/coreclr/tools/Common/Compiler/DependencyAnalysis/Relocation.cs

@@ -18,6 +18,10 @@ public enum RelocType
         // COFF relocation types
         IMAGE_REL_BASED_ADDR32NB             = 0x0B,   // The 32-bit address without an image base (RVA)
 
+        // Webcil Relocation Types
+        IMAGE_REL_BASED_WASM32_TABLE         = 0x0C,
+        IMAGE_REL_BASED_WASM64_TABLE         = 0x0D,
+
         // General relocation types
         IMAGE_REL_BASED_REL32                = 0x10,   // 32-bit relative address from byte following reloc
         IMAGE_REL_BASED_THUMB_BRANCH24       = 0x13,   // Thumb2: based B, BL
@@ -807,6 +811,12 @@ public static RelocType GetFileRelocationType(RelocType relocationType)
                 case RelocType.IMAGE_REL_BASED_THUMB_MOV32:
                     return relocationType;
 
+                case RelocType.WASM_TABLE_INDEX_I32:
+                    return RelocType.IMAGE_REL_BASED_WASM32_TABLE;
+
+                case RelocType.WASM_TABLE_INDEX_I64:
+                    return RelocType.IMAGE_REL_BASED_WASM64_TABLE;
+
                 default:
                     return RelocType.IMAGE_REL_BASED_ABSOLUTE;
             }